Known in the art is an internal combustion engine having arranged in an engine exhaust passage an NOx storing catalyst which stores NOx contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releases the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich. In this internal combustion engine, the NOx produced when burning fuel under a lean air-fuel ratio is stored in the NOx storing catalyst. On the other hand, when the NOx storing capability of the NOx storing catalyst approaches saturation, the air-fuel ratio of the exhaust gas is temporarily made rich and thereby the NOx is released from the NOx storing catalyst and reduced.
However, the fuel contains sulfur. Therefore, the exhaust gas contains SOx. This SOx is stored in the NOx storing catalyst along with the NOx. However, this SOx is not released from the NOx storing catalyst just by making the air-fuel ratio of the exhaust gas rich. Therefore, the amount of SOx stored in the NOx storing catalyst gradually increases. As a result, the amount of NOx which can be stored ends up gradually decreasing.
Known in the art therefore is an internal combustion engine having an SOx trap catalyst arranged in the engine exhaust passage upstream of the NOx storing catalyst so as to prevent SOx from being sent to the NOx storing catalyst (see Japanese Unexamined Patent Publication (Kokai) No. 2005-133610). In this internal combustion engine, the SOx contained in the exhaust gas is trapped by the SOx trap catalyst. Therefore, the SOx is prevented from flowing into the NOx storing catalyst. As a result, the storing capability of the NOx can be prevented from dropping due to storage of SOx.
In this regard, so long as this SOx trap catalyst is working under its predetermined use, it will continue to trap the SOx contained in the exhaust gas within a predetermined guaranteed driving distance. However, for example, should the vehicle be driven when the user mistakenly use fuel containing a high concentration of sulfur or when the predetermined guaranteed driving distance is exceeded, there is a problem that over the allowable amount of SOx will be sent to the SOx trap catalyst and therefore the SOx trap catalyst will no longer be able to trap SOx.
Considering this problem, when a large amount of SOx is trapped in the SOx trap catalyst, it becomes necessary to make the SOx trap catalyst release SOx to restore the SOx trapping capability of the SOx trap catalyst.